feat(config): add toolHintMaxLength to control tool hint truncation

Add  to  config (default: 40, range: 20-500).
Controls how many characters of tool hints are shown in progress updates
(e.g. '$ cd …/project && npm test').

Set to 120+ to see full commands instead of truncated hints:

```json
{
  "agents": {
    "defaults": {
      "toolHintMaxLength": 120
    }
  }
}
```

- Thread max_length through format_tool_hints → _fmt_known/_fmt_mcp/_fmt_fallback
- Make path abbreviation in _abbreviate_command proportional to max_length
- Add TestToolHintMaxLength test class with 5 tests
- All 41 existing tests pass

nanobot/config/schema.py

@@ -81,6 +81,13 @@ class AgentDefaults(Base):
     max_concurrent_subagents: int = Field(default=1, ge=1)
     max_tool_result_chars: int = 16_000
     provider_retry_mode: Literal["standard", "persistent"] = "standard"
+    tool_hint_max_length: int = Field(
+        default=40,
+        ge=20,
+        le=500,
+        validation_alias=AliasChoices("toolHintMaxLength"),
+        serialization_alias="toolHintMaxLength",
+    )  # Max characters for tool hint display (e.g. "$ cd …/project && npm test")
     reasoning_effort: str | None = None  # low / medium / high / adaptive - enables LLM thinking mode
     timezone: str = "UTC"  # IANA timezone, e.g. "Asia/Shanghai", "America/New_York"
     unified_session: bool = False  # Share one session across all channels (single-user multi-device)